10 Answers

I would say “Can I take a raincheck on that one?” Something like that. “Will you accept a raincheck” seems a little formal and stilted for casual conversation. “Will you accept a raincheck” seems more like something a clerk at a store would ask, when you were actually being offered an official store raincheck on a sale item and you were making a fuss about the item not being available.

Probably would be better to say “No thank you. I’m not interested now or ever.” The term “rain-check” implies that you will do it later. Which you apparently don’t want to do. But this doesn’t let the person know that they should look elsewhere for whatever it is they want done.
If it isn’t a friend, their hurt feelings wouldn’t matter to you. If it’s a friend, not being honest and giving the person the heads up to look elsewhere for their arrangements is not being a very good friend.

Can I take a rain check. But, as said above, you are implying you want to do it in the future. As @lillycoyote mentioned, you can still literally get rainchecks, a piece of paper, from stores when they have a coupon or offer and they run out of the product. They give you a raincheck that you can present to still get the better price when the goods come in even if the deal had expired. Some coupons and offers say, while supplies last, which eliminates the need to extend rainchecks to customers.